How about something like learning paintless dent removal, or a similar service. I believe a class in this wouldn't be too long and tool investment minimal. People will have accidents in winter time. Paint is very expensive (I know I am an auto body tech as my profession) so they may want to avoid repainting the more minor dents they get. It could also complement your detailing business if you go into that later. I do auto body on the side, besides doing it as a day job, and things are slow right now for me at home, fairly busy at work, but am hoping things pick up as winter rolls around people take their cars off the road for winter or have minor accidents they don't want to report to insurance (don't do insurance work at home). Another idea would be to see if you could get a job in an autobody shop. There is always one somewhere that will need someone to clean up cars after painting, pick up around the shop and empty garbage. It isn't great pay, but could get you some experience with using a buffer and wetsanding, fixing blemishes if you don't already. Just don't tell them you want to learn on their time then leave and start a mobile business.
Hope this helps.